It is proposed to re-evaluate the regenerative potential of severed axons of the spinal cord and corpus callosum using the female cat and rat. Possible role of progesterone will be explored. Treated and untreated subjects will be compared, using double blind procedures. Surgical techniques in the cat will reproduce those used previously by Clemente, et al. All experiments will be controlled histologically. There is reason to believe that regeneration of severed mammalian nerve fibers occurs with greater facility in females than in males. Particular attention will be given to the role of female sex hormones. Application of progesterone in suppressing immunity mechanisms and promoting tumor growth has been demonstrated in monkeys. Successful results in spinal cord regeneration with administration of the bacterial polysaccharide, Piromen, may have been related to change in ovarian structure induced by this pyrogenic preparation. Secondary constriction of the healing and regenerating spinal cord by connective tissue limited the beneficial effects of Piromen. Experiments in the corpus callosum may not be so limited by this factor. The use of rats as experimental animals will provide a control for earlier experiments by Freeman et al; most of whose studies lacked histological verification.